It is often desired to non-invasively stimulate or modulate neuronal firing in the brain by use of externally applied electric or magnetic forces.
Neurostimulation requires sufficiently large electric or magnetic force to provoke neuronal firing that would not otherwise occur, whereas neuromodulation is for controlling or influencing neuronal firing that occurs naturally (although often abnormally), and therefore employs forces of lesser magnitude. Both stimulation and modulation are used for research purposes, and modulation is particularly useful for therapeutic purposes such as for suppressing seizures in people with epilepsy.
The standard terminology used to refer to such methods does not distinguish between stimulation and modulation. Where electrical potentials are applied to the surface of the head to drive plate electrodes for injecting electrical currents into the head and brain, the methods are known as “transcranial current stimulation” or TCS; and where time-varying magnetic potentials are created outside the surface of the head for inducing electrical currents to flow inside the head and brain, the methods are known as “transcranial magnetic stimulation” or TMS. So, the terms “TCS” and “TMS” refer to neurostimulation or neuromodulation, or both, and for purposes herein, the term “stimulate” is intended to refer to either stimulating or modulating.
TCS methods are distinguished between those that inject DC currents, referred to as “transcranial direct current stimulation” or TDCS, and those that inject AC or pulsed currents, referred to as “transcranial alternating current stimulation” or TACS. Since TMS requires time-varying magnetic fields, there is nothing analogous to TDCS in TMS.
U.S. Pat. No. 7,190,826 describes a geodesic sensor net which may employed to provide a dense array of electrodes, e.g., 256, for use in TCS. The electrodes are electronically selected in desired current source/current sink (hereinafter “source/sink”) combinations at the head surface to provide for the stimulation/modulation. The electrodes may be replaced with, or used in combination with, magnetic coils for practicing TMS.
TMS methods employ one coil or one pair of coils which may be thought of as one-for-one substitutes for a source/sink pair of electrodes used in TCS. Currents flowing through the coils produce corresponding magnetic fields, and in particular, time-varying currents produce corresponding time-varying magnetic fields which induce electric currents to flow in the nearby circuitry defined by the neural pathways in the brain. So TMS serves the same purpose served by TCS.
The target of the stimulation/modulation is one or more “patches” of neurons, which are local groups of neurons that “fire together” and can therefore be treated as being single sources of electrical activity.
U.S. Pat. No. 6,594,521 describes a TCS method for determining how to stimulate a target patch of neurons in the brain, by replicating the potential distribution or lead-field projection that is measured at the head surface as a result of un-stimulated activity from the target, based on principles of Lorentz reciprocity.